1. Field of the Invention
The present invention relates to a piezoelectric device for converting an input mechanical quantity into an output electrical quantity, and vice versa.
2. Description of the Related Art
As generally known in the art, piezoelectric devices are typically used to convert an input mechanical stress into an output electric charge (voltage) and also to convert an input electric voltage into an output mechanical strain. In the latter case, the output strain can be used to generate mechanical force, displacement or vibration, and the device is sometimes called as an electrostrictive device. For the sake of convenience, the term "piezoelectric device" as used herein is to be interpreted in its broadest sense as encompassing an electrostrictive device as well. Also unless otherwise noted, the term "piezoelectric material" may be used in its broadest sense as encompassing an electrostrictive material as well.
Conventionally, there have been progressive demands for actuator devices capable of adjusting lengths and/or positions of an optical path on a submicron order, or for sensors capable of detecting fine mechanical displacement as an electric change. In order to satisfy these demands, developments have been made to realize improved actuators or sensors which utilize displacement due to a reverse piezoelectric effect or electrostrictive effect which is caused by an electric field applied to a piezoelectric or electrostrictive material, such as ferroelectric material, and vice versa. In the case of actuators, for example, developments are directed to achieve a compact and less-expensive arrangement of the device which stably operates at a low driving voltage and yet provides a high-speed response characteristic and a satisfactory operational reliability.
Known piezoelectric devices typically include a ceramic substrate having a cavity which is defined by a thin-walled region in the substrate. The thin-walled region of the substrate is provided thereon with a piezoelectric a transducer. Such transducer is comprised of laminated layers including a lower electrode layer, a piezoelectric or electrostrictive layer and an upper electrode layer which are formed one above the other. With such an arrangement of known devices, when a piezoelectric layer comprising a piezoelectric material as opposed to an electrostrictive material is formed between the two electrode layers and the upper and lower electrodes are applied with a voltage having a polarity which is same as that for performing a polarization treatment of the piezoelectric layer, a transverse effect of strain is induced by the applied electric field and causes the piezoelectric transducer to deflect toward the cavity. Alternatively, when an electrostrictive layer comprising an electrostrictive material is formed between the two electrode layers and the upper and lower electrodes are applied with a voltage, the electrostrictive transducer is caused to deflect toward the cavity, notwithstanding the polarity of the applied voltage.
When the above-mentioned known piezoelectric device is to be used as an actuator for a relay, the piezoelectric transducer is provided with a contact on its upper electrode layer. For driving the transducer in a direction toward the contact, i.e., away from the cavity, when the layer between the two electrode layers is comprised of a piezoelectric material as opposed to an electrostrictive material, the upper and lower electrodes have to be applied with a voltage having a polarity which is opposite to that used for a polarization treatment of the piezoelectric layer. In this instance, however, the voltage has to be limited to a level which is smaller than a coercive field which causes the reverse polarization. Thus, not only a sufficient displacement amount cannot be achieved, but also the displacement itself becomes unstable. Alternatively, when the layer between the two electrode layers is comprised of an electrostrictive material, it is impossible to perform a polarization treatment thereby to determine the polarization direction. Consequently, the driving direction of the transducer is always toward the cavity even when the polarity of the applied voltage is reversed, and it is thus impossible to drive the transducer in an opposite direction away from the cavity.
It is of course possible to cause displacement of the transducer in a direction away from the cavity, by utilizing a reaction force of the transducer as it returns to the initial position after being driven toward the cavity. In this instance, however, it is difficult to control the displacement amount and speed of the transducer. Also, the piezoelectric transducer can be driven in a direction away from the cavity, by forming the transducer on the inner surface of the thin-walled region in the substrate. However, particularly due to the cavity structure of the ceramic substrate, the transducer cannot be readily formed on the inner surface of the thin-walled region.
Besides, in the case of the above-mentioned arrangement of piezoelectric devices, due to possible manufacturing tolerance during the production steps, the piezoelectric transducer may be formed at a location slightly offset from the center of the thin-walled region of the ceramic substrate. In this instance, the displacement of the transducer is significantly influenced by the rigidity of the relatively thick-walled region of the substrate adjacent to the thin-walled region, and the displacement amount becomes smaller at a portion of the transducer which is situated closer to the thick-walled region. Therefore, such a piezoelectric device cannot effectively achieve a proper displacement amount even in combination with various auxiliary measures.